The designation makes the 801,000-acre (324,153-hectare) West Texas park, which borders the Rio Grande, one of only 10 places on the International Dark Sky Park list. It is one of only six at the Gold Tier level. Parks on the list have especially dark night skies and minimal light pollution, making stargazing easier.

Big Bend was found to have the darkest measured skies in the lower 48 states, and is now the largest park in the world with the Dark Sky designation. For those who seek to protect the park, the news comes as a welcome step forward.

The designation “acknowledges how important the dark sky is. There a lot of times light pollution isn’t considered a form of pollution, but if you are an astronomer and you want to experience the night sky as it was hundreds of years ago” you don’t want light pollution to interfere, said Evelyn Merz, conservation chair for the Sierra Club’s Lone Star chapter in Texas. “Normally there should be a dark, clear sky.”

Stephanie Kodish of the National Parks Conservation Association agrees. “I think that the designation is amazing. Designations like that honor the resources at the park” and highlight them for the international community, she said.

But while efforts to cut down on light pollution might help make the skies ripe for viewing stars, Big Bend still faces trouble with air pollution and visibility, often due to coal-burning power plants in East Texas and in the eastern United States, as well as from across the border, in Mexico.

Big Bend’s website educates visitors about the impact that the haze, created when sunlight hits pollution particles in the air, has on visibility. Visibility effects include a “reduction of the average natural visual range from about 160 miles (258 kilometers) (without the effects of pollution) to about 70 miles (113 kilometers) because of pollution at the park” and “frequent impairment of scenic vistas by haze.”

“Sulfate particles are the single largest contributor to haze at the park,” according to the site. “Sources of this secondary pollutant include coal-fired power plants, metal smelters, refineries, other industrial processes, and volcanoes.”

“The park is severely polluted and is unfortunately in a state and a region with many sources of pollution,” the NPCA’s Kodish said.

Although the U.S. Clean Air Act requires the state of Texas and the Environmental Protection Agency to work to reduce and eliminate the haze, the Sierra Club says that under a proposed new rule, Texas’ coal plants could purchase emissions allowances from other states, allowing them to escape a requirement to install more modern pollution controls.

The Sierra Club and the NPCA are among the environmental groups now pushing the EPA not to “let big polluters off the hook,” and are calling on people to protest the rule by filing comments before February 28.

“What needs to happen is forces that are polluting our parks like coal plants would have to install modern pollution controls, which isn’t a big ask,” said the Sierra Club’s Stephanie Cole, who works on its Beyond Coal initiative. “What the EPA has proposed for this rule is to exempt some of our oldest and dirtiest plants. We’re asking them to improve this rule to require them to install modern pollution control technologies.”

“Some of these power plants out in East Texas — they’re consistently ranked among the dirtiest in the country,” Cole said. “We feel like this could be giving these power plants a free pass.”

The Sierra Club’s Merz highlights that although Big Bend and the surrounding areas had to do a lot of work to clean up light pollution, air pollution is more difficult to tackle.

“The haze or clear sky is much more challenging,” she said. “The regional haze problem is not there every day — and thankfully it isn’t — not only because you don’t want have it there every day, but because on the days when you don’t have it, you realize what you’re missing when it is there.”

In the meantime, Merz said the Dark Sky designation “brings home what a great national resource we have in the clear skies over Big Bend.”